Control servo system



N 1961 T. w. JENKINS, JR., ETA]. 3,008,072

CONTROL SERVO SYSTEM 2 Sheets-Sheet 1 Filed Jan. 50, 1959 Nov. 7, 1961 T. w. JENKINS, JR, ET AL 3,008,072

CONTROL SERVO SYSTEM 2 Sheets-Sheet 2 Filed Jan. 30. 1959 United States Patent 3,008,072 CONTROL SERVO SYSTEM Theron W. Jenkins, Jr., Fort Washington, and Justus C. Barber, King of Prussia, Pa., assignors to Leeds and Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania.

Filed Jan. 30, 1959, Ser. No. 790,265 18 Claims. (318-28) This invention relates to automatic control of variable physical characteristics, such for example as temperature or pressure or other conditions, and more particularly to an electrical control system which provides proportional position control plus reset action and rate action by means of directional change-signals, as in the form of pulses of variable duration and/ or of variable frequency.

The present invention is to be contrasted with prior systems providing proportional position control action, reset action and rate action by reason of the fact that such prior art systems require more than one channel between the control system and the final control element. One such channel is utilized for the control of the final control element and the other to introduce into the control system a feedback related to the position of the final control element.

In accordance with the present invention, only a single channel is required between the control system and the drive unit for the final control element. There is not utilized a channel which provides signals indicative of the position of the final control element. Nevertheless, in accordance with the present invention, there are retained control operations including the proportional, reset, and rate actions mentioned above.

In carrying out the present invention in one form thereof, there is provided a measuring network for developing a control signal in accordance with the magnitude of a condition, which may be pressure, temperature, the pH of solutions, and the like. The control signal will be of one direction or the other, as polarity or phase, dependent upon Whether the magnitude of the controlled variable has deviated in one direction or the other from a predetermined value, generally referred to as the set point. In response to the control signal, circuit controlling means, such as relays, are utilized to complete energizing circuits for the actuating means of the final control element for producing a control action to increase or to decrease the magnitude of the controlled variable to return it toward its set point. There are associated with the circuit controlling means charging circuits for an electrical storage means, such as a capacitor, for supplying current to that capacitor in one direction when the deviation in the controlled variable is in one direction and for supplying current to the capacitor in the opposite direction when the deviation is in the opposite direction. That capacitor is connected into the measuring circuit for development therein of a signal to balance the control signal. Accordingly, the time of closure of the circuit controlling means is made dependent upon the time required for the capacitor to acquire a charge equal to the control signal. As will be later explained at length, there are introduced into the operation of the system the aforesaid proportional, reset, and rate actions, as may be desired, by so modifying the control and rebalancing signals as to produce such actions.

The present invention provides great flexibility in the control actions which may be introduced to meet the requirements of widely differing installations. The control actions are individually adjustable in magnitude. These features will be set forth, together with further objects and advantages of the invention, in the follow- 3,008,072 Patented Nov. 7,, 1961 ing more detailed description to be taken in conjunction with the accompanying drawings, in which:

FIG. 1 diagrammatically illustrates a system embodying one form of the invention;

'FIG. 2 diagrammatically illustrates some of the components of an amplifier utilized in the system of FIG. 1; and

FIG. 3 diagrammatically illustrates the manner in which FIG. 1 may be modified to include therein totalizing control actions useful in the control of a plurality of final control elements.

Referring to FIG. 1, the invention has been illustrated in one form as applied to a system for the control of the temperature of a compartment 10 heated by any suitable means, as for example, by a steam pipe 11, the steam flow thereto being under the control of a regulating valve 12, the position of which can be adjusted by a motor 13. Where the controlled condition is the temperature of the compartment 10, a thermocouple 14 responsive to that temperature applies to a measuring system 16 a voltage which causes a detector 17 to operate the contact of -a rebalancing slidewire 18 and to produce on a scale 19 and on a chart 20 an indication of the temperature of compartment 10. The measuring system 16 including the detector 17 is preferably of the electronic type, as illustrated in Williams Patent No. 2,113,164.

In order to produce a signal as represented by a voltage, E of sense and magnitude varying with the sense and extent of deviation of the controlled variable from a predetermined value, its set point, the measuring instrument including the detector 17 is mechanically connected, as indicated by the broken line 121, to the movable contact 22a of a slidewire resistor v22. :The resistor 22 is included in a network energized from a suitable source, as a battery 23. The network includes equal-valued resistors 24 and 25 connected in series across the battery 23. A slidewire resistor 26 is connected at one end between the juncture of resistors 24 and 25 and at the other end to contact 22a.

It is to be understood that all slidewire resistors referred to herein may be mounted on rotatable supports carried by mounting shafts for rotation relative to fixed stationary contacts. For convenience, the slidewire contacts have been illustrated in the drawings as relatively adjustable with respect to the slidewire resistors. The slidewire cont-act 22a is shown as adjustable by detector 17 while its associated slidewire resistor 22 is bodily rotatable relative to contact 22a in order to establish the set point.

For the apparatus of FIG. 1, when the temperature of compartment 10 is at the set point, the contact 22a is in the middle of the slidewire resistor 22 indicated by arrow S and the voltage E is of zero magnitude. By rotating the slidewire resistor 22, relative to slidewire resistor 18, it is possible for the voltage E to be made zero for any position of contact 22a and thus to establish a set point corresponding with any desired temperature in compartment 10. The contact 22a is shown to the right of the set point. As the temperature of compartment 10 decreases, the contact 22a will be moved toward the set point midway of slidewire 22. That point, however, can be bodily moved, for example, by rotating slidewire 22 until arrow S corresponds with the illustrated position of contact 22a to establish the set point at the existing temperature. As the midpoint of slidewire 22 is moved in a counterclockwise direction, set points will be established for higher temperatures within compartment 10, while clockwise rotation of slidewire resistor 22 establishes lower set point temperatures.

It will be assumed that a rise in temperature of compartment 10 has resulted in relative movement of contact 22a from the set point S to the illustrated position. There immediately appears across conductors 30 and 31 an output voltage E of magnitude related to the deviation of the controlled variable from the set point. The magnitude of voltage E for a given deviation may be adjusted by the position of contact 26a relative to a slidewire resistor 26. The deviation voltage E is applied by conductors 30 and 31 to resistors 32 and 33. Due to current flow through them, a potential dilference or voltage E is developed across the resistor 32 which, it will be noted, is included in series in the input circuit to a detector shown as an amplifier 35. Also in series circuit relation in the input circuit of amplifier 35 is an adjustable rate resistor 36 which through a rate capacitor 37 and a switch 38 is connected across resistor 33. By reason of the described connection of slidewire resistor 36 in series with capacitor 37, there is developed across resistor 36 in the input circuit of detector 35 a voltage E which varies in accordance with the rate of change of the controlled variable. Also included in series-circuit relation with the input of amplifier 35 is a capacitor 40. As will later be explained, the capacitor 40 develops in the input circuit a balancing potential difference or a voltage E The voltage E is varied in magnitude and direction until it balances the algebraic sum of said voltages E and E in the input circuit of amplifier 35.

When there is voltage unbalance in the input circuit of amplifier 35, there will be an output by way of one or the other of its output circuits respectively including conductors 41 and 42. Each output circuit includes a common return conductor 43. With the assumption of the increase of temperature in compartment 110 which gives rise to an unbalance voltage, there will be produced an output from amplifier 35 by way of conductors 41 and 43 for energization of the operating coil of a relay 44. As shown, the output circuits from amplifier 35 receive their energization from the secondary winding of a transformer 46 having its primary winding connected across alternating-current supply lines 47 and 48', the conductor 43 being connected to the mid-tap of the secondary winding. Relay 44, when energized, operates to open its 1) contacts, to close its a contacts and to close its contacts. The closing of the circuit through the a contacts of relay 44 completes an energizing circuit tor a motor winding 13a of motor 13 for rotation in a direction which, through the mechanical connection indicated by broken-line 49, operates valve 12 toward closed position to reduce the temperature of compartment 10. This circuit can be traced from supply line 47 by Way of a mechanically operable switch 50 illustrated in its A or automatic position, normally closed b contacts of relay 45, conductor 53, the a contacts of relay 4'4, switch 51 in its A or automatic position, and by way of the motor winding 13a and conductor 48a to the other supply conductor 48. The actuator shown as the motor 13 is thereupon energized to adjust the final control means or valve 12 toward closed position.

As the motor 13 is energized by closure of the circuit through the a contacts of relay 44, the capacitor 40 is connected to a charging circuit through the c contacts of relay '44. The polarity is such that current flows to the capacitor to increase its charge and to increase its voltage or potential diiference which in the input circuit of amplifier 35 opposes the algebraic sum of voltages E and E The motor -13 will be energized for operation until the voltage E increases to a magnitude equal to the algebraic sum of voltages E and E The source of charging voltage for the capacitor 4i) has been illustrated as a battery 54 across which there is connected a slidewire resistor 55 having a contact 55a which by its position relative to slidewire 55 provides adjustment of the magnitude of the charging voltage for capacitor 40. Thus with the 0 contacts of relay 44 closed, the charging circuit for capacitor 40 may be traced from the positive side of battery 54, slidewire 55, its contacts 55a, resistor 56, the c contacts of relay 44, conductors 5759, a switch 60, the capacitor 40, conductors 61 and 62, and to the juncture 63 between resistors 64 and 65, one of which, the resistor 64, is connected at one end to contact 550, and the other of which resistor 65 is connected at one end to the negative side of battery 54. The aforesaid charging circuit is completed from juncture 63 by [way of resistor 65 to the lower end of slidewire 55 and to the negative side of battery 54.

It is to be observed that there is a discharge circuit for capacitor 46 provided by a variable resistor 66 connected directly across the capacitor 40 by way of a switch 67. The variable resistor 66 in conjunction with capacitor 40 provides reset action in a manner which will be later explained.

With the above introductory explanation of the in vention in mind, it is to be noted that the operation of the motor and the rebalancing of the input circuit of the amplifier 35 are provided in the absence of any communication channel, mechanical or electrical, extending from the final control element, the valve 12, to the control network, the only connection to valve 12 being the mechanical connection from motor 13. In practice, the motor 13 will be mounted directly adjacent the final control element or valve 12. There will only extend to the motor the conductors 71, 72 and 48a which have been shown as including broken-line sections in FIG. 1 to illustrate the fact they may be of much greater length than appears in that figure. Despite the remote location of the motor 13 and the final control element 12, all of the desired control actions are realized in accordance with the present invention and as will now be demononstrated.

For a simplified explanation of the proportional action, it will now be assumed that the switches 38 and 67 have been moved to their open positions to render ineflective the rate capacitor 37 and the reset resistor 66. Accordingly, the only signal voltage introduced into the input circuit of the amplifier 35, due to change in the controlled variable from the set point, will be the voltage E As soon as this voltage E appears, due to an increase in temperature of compartment 10 from the set point, the relay 44 closes its 0 contacts, and capacitor 40 begins to charge with the resultant development of a balancing voltage E The magnitude of voltage E will for a given setting of slidewire contact 26a depend solely upon the magnitude of the deviation of the condition of the controlled variable from the set point. When suflicient time has elapsed for voltage E to become equal to voltage E the relay 44 is deenergized to open the energizing circuit for motor 13 and the charging circuit for capacitor 40. Thus, the motor 13 will be energized for a time interval corresponding with the magnitude of the aforesaid change from the set point. This means that the extent of the adjustment of the valve 12 in a closing direction will correspond with the extent of the aforesaid deviation.

The position of valve 12 and the motor 13 then remains fixed as long as no change occurs in the voltage E due to a change in the temperature of compartment 10. However, in response to the closing of the valve 12 by the proportional control action above described, the temperature of the compartment 10 will decrease from its previously assumed high value. This decrease in temperature will reduce the magnitude of the voltage E and there will no longer be a voltage balance in the input circuit of the amplifier 35. Since the voltage B; will then be larger than the voltage E the amplifier 35 through its output circuit 42, 43 will energize relay 45 for energization, through its a contacts, of the motor winding 13b for adjustment of the valve 12 in a valve-opening direction. Simultaneously with the energization of the motor winding 13b, relay 45 through its c contacts completes a charging circuit spasms for capacitor 40 through a resistor 73. It is to be noted that when the contacts of relay 45 are closed, the charging circuit for capacitor 40 is traced from the positive side of battery 54 by way of slidewire resistor 55, slidewire contact 55a, a resistor 64, conductors 62 and 61, the capacitor 40, switch 60, conductors 59 and 58, the c contacts of relay 45, and by way of resistor 73 to the negative side of the battery 54. Thus, the direction of current flow to the capacitor 40 is opposite to that produced by closure of the 0 contacts of relay 44. By the above action, the charge initially acquired by capacitor 40 is reduced resulting in a decrease in the voltage E which will eventually again balance the newly established lower magnitude of the voltage E During the time that the voltage E is being reduced to equality with the voltage E the motor 13 is operating the valve 12 in the valve-opening direction.

Thus, the foregoing adjustments of valve 12 correspond with a proportioning control action. The magnitude of the proportioning control action, generally referred to as the proportional band, determines the extent of adjustment of valve 12 for a given deviation of the controlled variable, the temperature of compartment 10. The proportional band may be readily adjusted by varying the position on slidewire 55 of contact 55a. This will change the magnitude of the voltage applied to the charging circuit of capacitor 40 and will thereby control the time required for voltage E to build up to equality with voltage E A like result may be accomplished by varying the position of the contact 2611 on slidewire 26, since this will change the magnitude of voltage E, for a given deviation of the controlled variable from the set point. In practice, the contacts 26a and 55a may be ganged together for concurrent adjustment, one in one direction and the other in the opposite direction.

Further modification of the proportional action may be achieved by adjusting the resistance of the charging circuit of capacitor 40 as by varying the value of resistor 56. The resistor 56 may have a value differing from the resistor 73. The resistor 73 determines the time constant of the charging circuit during operation of the motor 13 in the opposite direction. It is appropriate to here point out that in accordance with the present invention, the proportional action for deviation of the controlled variable in one direction may substantially dilfer from the proportional action when the deviation is in the opposite direction.

It will now be assumed that the switch 38 has been returned to its illustrated closed position. With capacitor 37 again in the circuit, it will be observed that charging current for that capacitor can flow by way of resistor 36. Any current flow through resistor 36 produces a potential difference of a voltage E in the input circuit of amplifier 35. If the deviation voltage E be changing, the magnitude of the voltage E will have a magnitude related to the rate of change of voltage E. This result meets the requirements of rate action, that is to say, the magnitude of the voltage E will depend upon the rate of change of the voltage E The effect of introducing the voltage E, into the input circuit of the amplifier is to require a different magnitude of the balancing voltage E, as developed by the capacitor 40 in order to produce a voltage balance in the input circuit of amplifier 35. Thus, when the temperature of compartment is above the set point and increasing, the voltage E will be in aiding relation to voltage E and the relay 44 will be energized for a longer time interval corresponding with the increased time required to charge capacitor 40 until voltage E balances the sum of voltages E and B In this manner the time of closure of relay 44 and the resulting position of the valve 12 includes a function corresponding with the rate of change of the controlled variable from the set point.

Reset action is introduced with switch 67 in its illustrated position to complete a discharge circuit for capacitor 40 through the resistor 66. The reset action is attained as follows. As soon as, and whenever, the voltage E following an increase in the temperature of compartment 10 above the set point is equal to the algebraic sum of voltages E and E the relay 44 is deenergized and so is the motor 13. As soon as the relay 44 opens, through its c contacts, the charging circuit for capacitor 40, that capacitor begins to discharge through reset resistor 66. The voltage E decreases. Accordingly, a difference voltage appears at the input of the amplifier 35 and relay 44 is again energized to complete the charging circuit for the capacitor 40 to reestablish therein that part of the charge dissipated through reset resistor 66. This action is a cyclic one, with corresponding incremental adjustments of motor 13 and of valve 12. As the discharge of capacitor 40 will be more rapid for larger magnitudes of the voltage E the average rate of adjustment of motor 13 will be greater for larger deviations of the temperature of compartment 10 from the set point. The sum of such adjustments introduces a control action related to the magnitude and duration of voltage E due to the deviation of the controlled variable from the set point. Thus, it will be seen that the reset resistor 66 introduces a modified operation of the system as a whole corresponding with the requirements of reset action. It has now been demonstrated that through the action of relay 44, its c contacts and the associated circuits, proportional, rate and reset control actions have been provided.

Had the deviation of the controlled variable, the temperature of compartment 10, been in the opposite direction, the polarity of the deviation voltage E would have been in the opposite direction and the amplifier 35 would through its output circuit 42, 43 have energized the relay 45 for energization through its a contacts of the motor winding 13b for adjustment of valve 12 in a direction to increase the flow of steam through pipe 11. Rate action is produced by rate capacitor 37 and rate resistor 36 and reset action is produced by reset resistor 66 in manner described above.

The present system lends itself to use with other features, such for example, as by the inclusion of signal lights 75 and 76 respectively of different colors, such as amber and red, to indicate by the state of their energization, the direction of adjustment of the final control element, the valve 12. Additionally, the manually operable switches 50-52 may be moved from their A to their M positions. When in their M or manual positions, 2. double-pole switch 77 may be manually operated to energize either of windings 13a and 13b for opening or closing valve 12 as may be desired. In this connection, it is to be noted that the relays 44 and 45 continue to be energized in accordance with the direction and amplitude of the algebraic sumof voltages E E and E Accordingly, the voltage B, will be developed during manual operation of direction and magnitude to balance the algebraic sum of voltages E and E This means that upon return of switches 50-52 to the automatic positions, there will be achieved what is known to those skilled in the art as bumpless transfer, that is to say, the position of valve 12 will not be radically changed upon return to automatic operation. Any change in position of valve 12 will be a controlled one in which there is absent a large proportional step.

Referring now to FIG. 2, the detector or amplifier 35 of FIG. 1 has been illustrated as including a vibrator 80 of the single-pole double-throw type, preferably polarized, and energized from the same alternating-current source of supply as transformer 46. When the vibrator contact is in its left-hand position, the algebraic sum of voltages E and E is applied to the input circuit of the first stage of the amplifier, which input circuit includes a coupling capacitor 81 and conductor 74. When the vibrator contact is in its right-hand position, there is applied to the input circuit including coupling capaci tor 81 and conductor 74 the voltage E developed across capacitor 40. Accordingly, the coupling capacitor 81 will develop in the input circuit an alternating-current signal of amplitude proportional to the difference between the voltage E and the algebraic sum of voltages E and E Its phase will be dependent upon the direction of the aforesaid difference. Accordingly, there will be produced from the amplifier 35 an output for energization of either of relays 44 or 45 of FIG. 1 depending upon the phase of the alternating-current signal. It is to be understood that the input circuit to the vibrator 80 may include said voltages E E and E in series for comparison with a reference or zero voltage, in which event the vibrator 80 would similarly produce an alternating current with an amplitude proportional to the difference and a phase in accordance with the direction of that difference. Input circuits, such as shown in FIG. 2, and of the type just mentioned have been illustrated as FIGS. 2 and 3 of Davis et al. Patent 2,830,245. A more detailed illustration of a suitable amplifier including the output circuits appears as FIG. of Davis Patent 2,797,291. The amplifier 35 selectively energizes relays 44 and 45, depending upon whether the alternating current applied to the input of the amplifier by way of vibrator 80 and coup-ling capacitor 81 is of one phase or of opposite phase. When the alternating current is of one phase, one of the triode sections of the output tube 35a will be conductive and the relay connected in series therewith will be energized. When the input signal is of opposite phase the other relay will be energized through the remaining triode section of the output tube 35a. This phase selectivity is achieved by the provision of the transformer 46 supplied from the same alternating current supply which energizes the operating coil of vibrator 80. The secondary winding of transformer 46 has its mid-tap connected to the cathodes of the output tube 35a and the respective ends of the secondary winding are by way of the relays 44 and 45 connected by conductors 41 and 42 to the anodes of the respective triode sections of the output tube.

Before describing the manner in which the present control system lends itself to a totalizing of the control actions of a plurality of final control elements, there are a few more features of FIG. 1 to be discussed. Inasmuch as the several control actions have been attained by the utilization of the operation of the relays 44 and 45 in conjunction with the charging circuit for the capacitor 40, it will be seen that there is achieved a reset limiting action. A reset signal, as by continued operation of relays 44 and 45 to'maintain charge E on capacitor 40 to balance voltages E and E is maintained when the final control element is moved to one or the other of its limits and a limit switch (not shown) on the motor circuit 13 is opened. The charge on capaci tor 40 is independent of the position of valve 12, and is dependent only on the operation of relays 44 and 45, which continue to operate even when motor 13 is not operating due to opening of limit switches (not shown). Thus, the input circuit of the amplifier 35 will be held in a balanced condition ready to respond to any change toward the set point in the magnitude of the condition under control to move the valve 12 away from its limit position. In this manner, it will be seen that the present system lends itself to batch-types of processes in which the controlled variable may be away from the set point for substantial periods of time, as during startup. The described control action can be considered as limiting the shift of the proportional band to an extent that will maintain one edge of the proportional band at the value of the condition under control.

The system also lends itself to the introduction of control actions by other variables. As will be explained in connection with FIG. 3, such additional control action may be one of an anticipatory type and also of pulsating character. Such additional control action may also be of the continuous type, that is to say, as by a continuously varying voltage which may be introduced by way of input terminals 83, FIG. 1, for applying a voltage across a slidewire 84 which may be connected in series with reset capacitor 40 by operating switch 60 to its right-hand position. The introduction of a voltage between slidewire contact 84a and the conductor 58 will introduce the additional voltage in series with that developed by the capacitor 40 and will thus produce a proportional control action in accordance with the direction and amplitude of the introduced voltage. Thus, if the voltage introduced at the terminals 83 be proportional to the temperature of the steam in line 11, it will be seen that a proportional anticipatory action may be introduced, since there would then be included in the operation of the control system a corrective action in the position of valve 12 to change the setting of that valve to compensate for changes in temperature of steam line 11 before such changes adversely affect the control of the temperature of compartment 10.

If it be assumed that the magnitude of the temperature in the compartment 10 is steady at the set point, the voltages E and E will be zero and the voltage B, will similarly be maintained at zero. If the temperature of the steam in line 11 should change and by its change produce a voltage at the terminals 83 proportional to the change in temperature of the steam, the input circuit for the amplifier 35 will be unbalanced by the magnitude of this voltage. Accordingly, while B, and E remain zero, relay 44 or 45, depending upon the direction of change of temperature in steam line 11, will be energized to in turn energize motor 13 for rotation in a corrective direction and simultaneously to complete a charging circuit for the capacitor 40. As the charge on capacitor 40 builds up, the voltage across the capacitor will become equal and opposite to the change in voltage introduced by slidewire resistor 84 to rebalance the input circuit for the amplifier 35. At this time the motor '13 is deenergized and the charging circuit for capacitor 40 is opened. The valve will have been positioned by an amount determined by the magnitude of the change in voltage introduced by slidewire 84. Inasmuch as the voltage across capacitor 40 is then equal and opposite to the voltage introduced by slidewire resistor 84, there is no net voltage between the conductors 62 and 59. Therefore, even though the switch 67 be in its closed position, there will be no reset action because no voltage exists across the reset resistor 66 to produce a discharge flow of current.

Those skilled in the art will understand that compartment 10 is representative of a simple type of equipment whose temperature is to be controlled. The valve 12, instead of controlling the fiow of steam, may be a fuel burner valve, and in general the valve 12 is but one form of a final control element or condition-varying or control agent manipulating means to which the present invention is well suited.

Referring now to FIG. 3, there have been illustrated a plurality of final control elements 12A- 12C respectively adjusted by motors 13A-13C. A boiler furnace with multiple burners may be taken as illustrative of an application Where multiple final control elements are utilized. For such a system, a single thermocouple, such as the thermocouple 14 of FIG. 1, may be responsive to boiler steam temperature. As an alternate, steam pressure may also be utilized as the controlled variable. In FIG. 3, there have been illustrated the relays 44 and 45, together with the conductors 41-43 of FIG. 1. There have also been illustrated conductors 58 and 62 extending from the reset capacitor 40 and the reset resistor 66 of FIG. 1. Accordingly, it will be seen that FIG. 3 is but a fractional illustration of a system which includes the features of FIG. 1 including the amplifier 35, the measuring system, and that part of the control system utilized for the development of voltages E and E If but a single final control element is to be manipulated, one of the douple-pole, single-throw switches 91, 94 or 95, such for example as switch 91, may be moved to its 9 closed position for energization of relays 92 and 93 under the control of the relays 44 and 45. Thus, each time relay 44 is energized, the relay 92 will be energized for energization of motor 13A in one direction. Energization of relay 45 will energize relay 93 for energization of motor 13A in the opposite direction. The contacts of relay 92 complete a charging circuit for capacitor 40 through resistor 56a which in function corresponds with that of resistor 56 of FIG. 1. Through the 0 contacts of relay 93, there is completed a charging circuit of opposite polarity through a resistor 73a which corresponds in function with the resistor 73 of FIG. 1.

As thus far described, the operation will not differ from that of FIG. 1 for the control of the single motor 13A and its final control element 12A, providing, of course, resistors 56a and 73a correspond in value with resistors 56 and 73.

By closing manually operable, double-pole, singlethrow switches 94 and 95, relays 96 and 97 additionally are energized upon closure of the a contacts of relay 44, and relays 98 and 99 will be additionally energized upon closure of the a contacts of relay 45. The relays 96 and 97, through their 0 contacts, are eifective to connect into the charging circuit resistors 56b and 560, respectively. Similarly, the 0 contacts of relays 98 and 99 are elfective to include in the other charging circuit the resistors 73b and 730. With the resistors 56a-56c connected in parallel, it will be seen that the time constant of the charging circuit or capacitor 40 will be reduced with an accompanying increase in charging current. The capacitor 40 will be more rapidly charged. Accordingly, the motors 13A-13C will be energized for lesser periods of time when all three are under the control of the system than would be the case it but a single motor or a lesser number than the total number of motors were placed under the control of the system. A like result is achieved by the connection of resistor 73a-73c in parallel with each other in the charging circuit of capacitor 40.

It will be noted that limit switches are provided for the several contactors or relays controlling the motors 13A-13C. When a final control element is moved to one or the other of its limits, one or the other of the limit switches associated with that final control element is opened, thereby interrupting the circuit to one or the other of the control relays to remove from the charging circuit of the capacitor the corresponding resistors. Thus, when limit switches 105 and 106 are operated by the motor 13A, they open the energizing circuits to relays 92 and 93. Limit switches 107 and 108 similarly function in conjunction with relays 96 and 98, and limit switches 109 and 110 have like functions for opening the circuits of relays 97 and 99. Thus, when a final control element has reached one or the other of its limits, there is no longer modification of the changing circuit of the capacitor 40 by the relay associated with that final control element which has then reached one of its limits of operation.

Thus, for a given value of E the change in heat input to the process will be the same regardless of the number of final control elements that are being adjusted and without requiring any adjustment in the control circuit.

While in the discussion of FIG. 1 it was shown that anticipatory control could be introduced into this controller when the control signal was in the form of a voltage of varying magnitude, it is also possible to introduce anticipatory control where the auxiliary signal is in the form of raise or lower pulses such as from a controller similar to the one of this invention. FIG. 3 discloses how these pulse-type auxiliary control signals may be used with this controller, it being understood that such auxiliary control might also be applied to the controller of FIG. 1 in which there is but a single final control element shown. The pulse-type of signals as explained in Bristol Patent 2,681,-

418 are widely used in the control of generation. They are frequently of variable length and of variable frequency. If such pulse-type of signals are developed by the auxiliary control 102, they will produce energization of one or the other of relays 101 and 103. The relay 101 in completing the circuit through resistor 104 to the capacitor 40 causes a charging current to flow through the capacitor 40 to produce a voltage difference across the capacitor with the upper terminal of capacitor 40 positive relative to the lower terminal. The charge accumulated by capacitor 40 and hence the voltage across it is dependent upon the length of time that the relay 101 is energized by the auxiliary control 10 2. This voltage across capacitor 40 which appears in the input circuit to amplifier 35 will produce operation of relay 44 and one or more of relays 92, 96 and 97 which through their c contacts closes a charging circuit for capacitor 40 in the reverse sense to the circuit established through the resistor 104. The closure of the c contacts of the relays 92, 96 or 97 will persist until the charge on capacitor 40 has been reduced to make voltage E equal to the algebraic sum of voltages E and E The amplifier 35 (FIG. 1) then deenergizes the relay 44. During the time that re lays 92, 96 and 97 were energized, their respective motors 13A-13C adjusted the final control elements 12A-12C to produce a proportional control effect on the final control elements related to the length of the pulse applied to the relay 101 by the auxiliary control 102-. In the event that the control system be responding to a deviation in the condition under control at the time that relays 101 or 103 are energized, the two actions are superimposed and the motors ISA-13C operate for lengths of time modified due to the presence of the auxiliary control.

What is claimed is:

1. In a control system for maintaining the magnitude of a condition at a predetermined value, a circuit component, means for energizing said circuit component to produce a control signal of sense and magnitude corresponding with the sense and extent of deviation of the magnitude of said condition from said predetermined value, means including an electrical capacitor for producing a rebalancing signal, a detector having input and output circuits responsive to the difference between said control signal and said rebalancing signal, final control means for varying the magnitude of a condition-controlling effect, actuating means for said final control means, control means connected to said output circuit and responsive to the direction of the difference between said signals for controlling the energization of said actuating means for adjusting said final control means in a direction to return said condition to said predetermined value, and charging means for supplying to said capacitor a charging current of one polarity only when said actuating means is energized for adjustment of said final control means in one direction and for supplying to said capacitor a charging current of opposite polarity only when said actuating means is energized for adjustment of said final control means in the opposite direction.

2. The control system of claim 1 in which there is provided for said capacitor a discharge circuit including a reset resistor.

3. The control system of claim 1 in which there is provided rate responsive means for producing response by said detector to a signal component varying in accordance with the rate of change of said magnitude of said condition from said predetermined value.

4. The control system of claim 2 in which there is provided rate responsive means for producing response by said detector to a signal component varying in ac cordance with the rate of change of said magnitude of said condition from said predetermined value.

5. The control system of claim 1 in which said charging means for supplying to said capacitor said charging currents includes resistors for establishing time constants of differing magnitude when supplying charging current in one or the other direction.

6. The control system of claim 1 in which said charging means includes charging circuits including resistors, and means for changing the resistance of said charging circuits for establishing a time constant of one charging circuit differing from the time constant of the other charging circuit.

7. The control system of claim 6 in which said final control means includes a plurality of final control elements and in which said actuating means includes an actuator for each said final control element, and in which said means for changing said time constants of said charging circuits for said capacitor includes a plurality of resistors for each of said charging circuits in number corresponding with the number of said final control elements.

8. The control system of claim 7 in which there are provided circuit controllers for said plurality of resistors respectively to include in said charging circuits said resistors in number corresponding with the number of actuators energized under the control of said output circuit.

9. The control system of claim 1 in which there is provided an additional circuit component connected to said capacitor for introducing into said input circuit an additional signal, thereby to modify the voltage required of said capacitor to balance said control signal.

10. The control system of claim 1 in which there is provided for said capacitor a discharge circuit including a reset resistor, and an additional circuit component connected to said capacitor for introducing into said input circuit an additional signal, thereby to modify the voltage required of said capacitor to balance said control signal, said additional circuit component being connected in series with said capacitor and said reset resistor.

11. The control system of claim 1 in which said charging means for said capacitor includes charging circuits, and means responsive to the sense of an auxiliary signal for modifying one of said charging circuits relative to the other of said charging circuits for unbalancing the input of said detector for energization of said actuating means in a direction for adjustment of said final control element in a direction to anticipate a change of said condition from said predetermined value.

12. The control system of claim 11 in which said means for modifying said charging circuits includes resistors and relays, each said relay being energized in accordance with an auxiliary signal of one sense or of opposite sense for connecting said resistors to said charging circuits.

13. The control system of claim 12 in which said final control means includes a plurality of final control elements and in which said actuating means includes an actuator for each said final control element, and in which there is associated with each said charging circuit a plurality of resistors corresponding in number With the number of said final control elements.

14. The control system of claim 13 in which said plurality of resistors each has associated therewith circuitcontrolling means for connecting them into said charging circuits upon energization of said actuators and for disconnecting them from said charging circuits upon deenergization of said actuators.

15. A control system for maintaining the magnitude of a condition at a predetermined value, the combination of means including a sensitive element responsive to changes in the magnitude of said condition for producing a control voltage varying in magnitude and sense with the magnitude and sense of the deviation in the magnitude of said said condition from a predetermined value, an electrical capacitor, actuating means, detector means having an output circuit and an input circuit responsive to the algebraic sum of the voltages applied thereto for controlling the energization of said actuating means, final control means adjustable by said actuating means for varying a condition-controlling effect in direction to restore the magnitude of said condition to said predetermined value, charging circuits for said capacitor, means operable during the time said actuating means is energized in direction to change the magnitude of said condition in one direction for completing one of said charging circuits to said capacitor for acquirement by said capacitor of a charge developing a voltage opposite to that of said control voltage, and when said final control means is energized for changing the magnitude of said condition in an opposite direction for completing the other of said charging circuits to said capacitor for changing said charge in an opposite direction, and means connecting said capacitor into said input circuit for rebalancing that circuit after unbalance thereof by said control voltage.

16. In a control system 'for maintaining the magnitude of a condition at a predetermined value, a detector having an output circuit and an input circuit, said input circuit having a circuit component for developing a control signal in said input circuit, means for energizing said circuit component to produce in said input circuit said control signal of sense and magnitude corresponding with the sense and extent of deviation of the magnitude of said condition from said predetermined value, an electrical capacitor in series-circuit relation with said circuit component in said input circuit for producing therein a rebalancing signal, final control means for varying the magnitude of a condition-controlling effect, actuating means for said final control means, control means connected to said output circuit and responsive to the direction of the difference between said signal for controlling the energization of said actuating means for adjusting said final control means in a direction to return said condition to said predetermined value, and charging means for supplying to said capacitor a charging current of one polarity when said actuating means is energized for adjustment of said final control means in one direction and for supplying to said capacitor a charging current of opposite polarity when said actuating means is energized for adjustment of said final control means in the opposite direction.

17. A control system for maintaining the magnitude of a condition at a predetermined value by operating a final control element in one direction or the other to vary in one direction or the other the magnitude of a condition-controlling effect, actuating means for said final control element, selector means for controlling the direction of operation of said actuating means, means operable independently of movement of said final control element and in response to operation of said selector means for changing the magnitude of a voltage in one direction upon actuation of said final control element in one direction and for producing a change in said voltage in the opposite direction upon operation of said actuating means in the opposite direction, means responsive to change in the magnitude of said condition for producing a voltage of one direction or the other when said magnitude changes in one direction or the other from said predetermined value, and means for detecting a difference between said voltages for controlling said actuating means in accordance with the sense and extent of said difference for controlling said selector means for movement of said final control element in one direction or the other to vary the magnitude of a condition-controlling effect in a direction and for a duration of time to return said magnitude of said condition to said predetermined value.

18. A control system for maintaining the magnitude of a condition at a predetermined value by operating a final control element in one direction or the other to vary in one direction or the other the magnitude of a conditioncontrolling efiect, actuating means for said final control element, selector means for controlling the direction of operation of said actuating means, means operable independently of movement of said final control element and in response to operation of said selector means for changing the magnitude of a signal in one direction upon actuation of said final control element in one direction and for producing a change in said signal in the opposite directlon upon operation of said actuating means in the opposite direction, means responsive to change in the magnitude of said condition for producing a second signal of one direction or the other when said magnitude changes in one direction or the other, and means for detecting a difference between said signals for controlling said actuating means in accordance With the sense and extent of said difference for controlling said selector means for movement of said final control element in one direction or the other to vary the magnitude of a condition-controlling effect in the direction and :for a duration of time to return said magnitude of said condition to said predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 

